Electrophotographic marking machine having an imageable seam intermediate transfer belt

ABSTRACT

Electrophotographic marking machines having imageable seam intermediate transfer belts. A toner image is produced on a charged photoreceptor. That image is then transferred onto a seamed intermediate transfer belt without regard as to the relative position of the seam. The seam region has good electrical property correspondence with a far region away from the seam and a lateral resistivity greater than 10 8  ohms/square. That transferred toner image is subsequently transferred to a fusing member for fusing.

FIELD OF THE INVENTION

This invention relates electrophotographic printing machines.Specifically, this invention relates to electrophotographic printingmachines having seamed intermediate transfer belts.

BACKGROUND OF THE INVENTION

Electrophotographic printing is a well-known and commonly used method ofcopying or printing documents. Electrophotographic printing is performedby exposing a light image representation of a desired document onto asubstantially uniformly charged photoreceptor. In response to that lightimage the photoreceptor discharges, creating an electrostatic latentimage of the desired document on the photoreceptor's surface. Toner isthen deposited onto that latent image, forming a toner image. The tonerimage is then transferred from the photoreceptor onto a receivingsubstrate such as a sheet of paper. The transferred toner image is thenfused with the substrate, usually using heat and/or pressure. Thesurface of the photoreceptor is then cleaned of residual developingmaterial and recharged in preparation for the production of anotherimage.

The foregoing generally describes black and white electrophotographicprinting machines. Electrophotographic printing can also produce colorimages by repeating the above process for each color of toner that isused to make the color image.

For example, the photoreceptive surface may be exposed to a light imagethat represents a first color, say black. The resultant electrostaticlatent image can then be developed with black toner particles to producea black toner layer that is subsequently transferred onto a receivingsubstrate. The process can then be repeated for a second color, sayyellow, then for a third color, say magenta, and finally for a fourthcolor, say cyan. When the toner layers are placed in superimposedregistration the desired composite color toner image is formed and fusedon the receiving substrate.

The color printing process described above superimposes the color tonerlayers directly onto a substrate. Other electrophotographic printingsystems use intermediate transfer belts. In such systems successivetoner layers are electrostatically transferred in superimposedregistration from the photoreceptor onto an intermediate transfer belt.Only after the composite toner image is formed on the intermediatetransfer belt is that image transferred and fused onto the substrate.Indeed, some electrophotographic printing systems use multipleintermediate transfer belts, transferring toner to and from the belts asrequired to fulfill the requirements of the machine's overallarchitecture.

In operation, an intermediate transfer belt is brought into contact witha toner image-bearing member such as a photoreceptor belt. In thecontact zone an electrostatic field generating device such as acorotron, a bias transfer roller, a bias blade, or the like createselectrostatic fields that transfer toner onto the intermediate transferbelt. Subsequently, the intermediate transfer belt is brought intocontact with a receiver. A similar electrostatic field generatingdevices then transfers toner from the intermediate transfer belt to thereceiver. Depending on the system, a receiver can be anotherintermediate transfer member or a substrate onto which the toner willeventually be fixed. In either case the control of the electrostaticfields in and near the transfer zone is a significant factor in tonertransfer.

Intermediate transfer belts often take the form of seamed beltsfabricated by fastening two ends of a web material together, such as bywelding, sewing, wiring, stapling, or gluing. While seamlessintermediate transfer belts are possible, they require manufacturingprocesses that make them much more expensive than similar seamedintermediate transfer belts. This is particularly true when theintermediate transfer belt is long. While seamed intermediate transferbelts are relatively low in cost, the seam introduces a discontinuitythat interferes with the electrical, thermal, and mechanical propertiesof the belt. While it is possible to synchronize a printer's operationwith the motion of the intermediate transfer belt such that toner is notelectrostatically transferred onto the seam, such synchronization addsto the printer's expense and complexity, resulting in loss ofproductivity. Additionally, since high speed electrophotographicprinters typically produce images on paper sheets that are cut from apaper “web,” if the seam is avoided the resulting unused portion of thepaper web must be cut-out, producing waste. Furthermore, even withsynchronization the mechanical problems related to the discontinuity,such as excessive cleaner wear and mechanical vibrations, still exist.However, because of the numerous difficulties with transferring toneronto and off of a seamed intermediate transfer belt, some of which arediscussed below, in the prior art it was necessary to avoid tonertransfer onto (and thus off of) the seam.

Acceptable intermediate transfer belts require sufficient seam strengthto achieve a desired operating life. While the desired operating lifedepends on the specific application, typically it will be at least100,000 operating cycles, and more preferably 1,000,000 cycles.Considering that a seamed intermediate transfer belt suffers mechanicalstresses from belt tension, traveling over rollers, moving throughtransfer nips, and passing through cleaning systems, achieving such along operating life is not trivial. Thus the conflicting constraints oflong life and limited topographical size at the seam places a premium onadhesive strength and good seam construction.

A prior art “puzzle cut” approach to seamed intermediate transfer beltssignificantly reduces mechanical problems by producing an improvedmechanical seam. U.S. Pat. No. 5,514,436, issued May 7, 1996, entitled,“Puzzle Cut Seamed Belt;” U.S. Pat. No. 5,549,193 entitled “EndlessSeamed Belt with Low Thickness Differential Between the Seam and theRest of the Belt;” and U.S. Pat. No. 5,487,707, issued Jan. 30, 1996,entitled “Puzzle Cut Seamed Belt With Bonding Between Adjacent SurfaceBy UV Cured Adhesive” teach the puzzle cut approach. While puzzle cutsreduce mechanical problems there remains other difficulties withtransferring toner onto and off of a seam of a seamed intermediatetransfer belt.

For transferring toner onto and off of a seam to be acceptable, thefinal image produced from across the seam must be comparable in qualityto images formed across the remainder of the belt. This is a difficulttask due to a number of interrelated factors. Some of those factorsrelate to the fact that the seam should not greatly impact theelectrostatic fields used to transfer toner. However, electrostatictransfer fields are themselves dependent on the electrical properties ofthe intermediate transfer belt. While this dependency is complex and amore detailed discussion of this subject is given subsequently, brieflythere are conditions where transfer fields are very sensitive to theresistivity and thickness of the materials used for the various layersof the intermediate transfer belt. Under other conditions theelectrostatic transfer fields are relatively insensitive to thosefactors. Similarly, there are conditions where the electrostatictransfer fields are very sensitive to the dielectric constants of thematerials used for the layers of the intermediate transfer belt, andother conditions where the electrostatic transfer fields are insensitiveto the dielectric constants. Therefore, to successfully transfer toneronto and off of a seamed intermediate transfer belt the electricalproperties across and around the seam should be carefully controlled toproduce a proper relationship with the remainder of the belt. Since theelectrical properties depend on the interrelated factors of seamgeometry, seam construction (such as adhesive beyond the seam), seamtopology, seam thickness, the presence of an overcoating, and variousother factors those factors should be taken into consideration for agiven application.

From above it can be seen that if toner is to be transferred onto andoff of a seam that the critical properties at the seam region must becontrolled such that the electrostatic transfer fields across the seamare close to those away from the seam. While conditions that achievethis are discussed in more detail later, generally those conditionsinvolve the use of “forgiving resistivity ranges.” However, it should benoted that one only needs to provide seam conditions that result in“sufficiently close” electrostatic transfer fields. Sufficiently closedepends on the tolerance of a given system to differences in theelectrostatic transfer fields. Experience shows that some systems cantolerate more than a 20% difference in the electrostatic transfer fieldswithout a significant difference in the final image. However, highquality color systems usually must have less than a 10% difference toavoid noticeable problems. However, “sufficiently close” is bestdetermined by experimentation.

Even if the electrical properties of a seamed intermediate transfer beltare suitable for producing acceptable images across the seam region,other problems remain. For example, with prior art seamed intermediatetransfer belts relatively poor cleaning and transfer around the seam isacceptable. However, if toner is being transferred onto and off of theseam region the seam must be properly cleaned. Thus, the toner releaseand friction properties across the seam region would have to becomparable to those of the rest of the belt. Furthermore, most prior artseamed intermediate transfer belts have a significant “step” where thebelt overlaps to form the seam. That step can be as large as 25 micons.Such a step significantly interferes with transfer and cleaning. Thus iftoner is transferred onto and off of the seam, the seam's friction,toner release, and topography are much more constrained than those ofother seamed intermediate transfer belts.

From above it can be seen that a seam's topography is very important ifone wants to transfer toner onto and off of a seam region withoutsignificant degradation of the final image. The seam topography includesnot only the seam itself, but also any overflow of the adhesive used inthe seam. This overflow can occur on both the toner-bearing side and theback-side of the belt. Adhesive overflow is important because the beltseam strength can depend upon on that overflow. However, excessiveoverflow increases various mechanical, electrical, and xerographicproblems. Furthermore, the adhesive's electrical properties remainimportant.

When attempting to transfer toner onto and off of a seam the seam'stopography introduces spatial disturbances that are convenientlyclassified as “short-wavelength” disturbances and “long-wavelength”disturbances. While these disturbances both relate to the mean distancebetween adjacent peak-to-valley spatial defects, short-wavelengthdisturbances are small, say less than 3 millimeters, whilelong-wavelength disturbance are large, say greater than 3 millimeters,.While both disturbances must be sufficiently controlled,short-wavelength disturbances usually require more stringent controlthan long-wavelength disturbances. Short-wavelength disturbances on thetoner-bearing side of the belt are usually much more significant than onthe back-side.

Short-wavelength disturbances include, for example, bumps, valleys orsteps, kinks or distortions, and peak-to-valley roughness. Such defectsare results of the seam type, adhesive overspill, seam manufacturing, orgrinding or polishing. One problem with short wavelength disturbances isthat they introduce small, unwanted air gaps at the transfer nips. Dueto belt stiffness some “tenting” occurs due to short wavelengthtopography, and the extra air gaps caused by the short wavelengthtopography can then extend quite far beyond the location of the peak tovalley distortion. The unwanted air gaps can be reduced by pressure inthe transfer nip. Thus a pressured transfer fleld generation device,such as a conformable bias transfer roller, is generally preferred overa pressureless transfer field generation device, such as a corotron.

Small, unwanted air gaps could be reduced by using an intermediatetransfer belt having a conformable overcoat. However, a conformableovercoat can introduce other problems, such as friction or poorelectrostatic toner release. Also, for very short-wavelengthdisturbances, such as a large bump at the seam, the pressure needed toeliminate unwanted air gaps is normally impractical even if aconformable overcoat is used.

On the toner-bearing side small, unwanted air gaps can significantlylimit electrostatic transfer fields due to Paschen air breakdown. Asknown in the art, for air gaps between about 5 microns and 100 micronsthe maximum field, E_(c), that can be supported before breakdown in anair gap d_(A) decreases with an increasing air gap. This is calledPaschen air breakdown and it can be approximately expressed as:E_(c)=[6.2 Volts/m+(312 Volts)/d_(A) ]. When an applied E-field in anair gap tries to go above E_(c), an air breakdown charge transfer occursthat limits the field to near or below E_(c). Since air gaps of 5 to 15microns can already be present near the edges of and within a tonerimage, extra air gaps will reduce the maximum E-field that can bepresent during electrostatic toner transfer of the toner. For example,if air gaps in a toner layer are about 15 microns, Paschen air breakdownwill limit the applied electrostatic fields to around 27 volts/micron.However, if an unwanted air gap of 10 microns is introduced by the seamthe total air gap increases to 25 microns and the transfer E-field willbe limited to around 18.7 volts/micron. While a desirable transferE-field depends on many factors, air gap transfer E-fields are typicallyabove 20 volts/micron and often above 35 volts/micron.

In addition to transfer problems, short-wavelength disturbances candegrade the effectiveness of cleaning systems. Blade cleaning systemstend to work better with very small short-wavelength disturbances. Forexample, short-wavelength disturbances of about 0.1 microns can resultin reduced friction between the blade and the cleaning surface, therebyhelping cleaning.

Therefore, when attempting to transfer toner onto and off of a seam theseam's topography should not introduce transfer nip air gaps abovearound 10 microns. Preferably unwanted air gap should be less thanaround 5 microns, and more preferably less than around 1 micron.

When attempting to transfer toner onto and off of a seam withoutseriously impacting the final image, the seam's long-wavelengthdisturbances also must be sufficiently controlled to produce anacceptable final image. Examples of unwanted long-wavelengthdisturbances include “belt ripple” or “belt waviness” longer than 3millimeters. Long-wavelength disturbances usually are less importantthan shortwave-length disturbances because a relatively low pressure ona belt can flatten long-wavelength disturbances. Thus it is preferableto use a pressured transfer field generation device, such as anip-forming bias transfer roller. Also, it is beneficial to tension thebelt in cleaning zones such that the belt is relatively flat.

While small disturbances can be significant on the toner-bearing side ofa belt, larger backside disturbances can usually be tolerated. First,this is because air gaps introduced by back-side disturbances do notusually cause unwanted air gaps on the toner-bearing side of the belt.Therefore back-side induced Paschen air breakdown is not a major issue.Second, since good back-side cleaning is usually not required thetopography constraints related to cleaning are typically not an issue.Finally, for a conformable belt, belt conformance can prevent gaps onthe back-side of the belt from being a significant problem. In general,back-side topography should not introduce air gap higher than 10microns, and preferably it should be less than 5 microns.

While seamed intermediate belts without an overcoat are relatively lowcost and relatively simple to manufacture, an overcoat on the tonerbearing surface can insure that the seam region has the same tonerrelease and friction properties as the rest of the belt. This enables awider range of adhesives to be used. Therefore, seamed intermediatetransfer belts typically include a substrate layer and an overcoatformed from one or more overcoating layers. Those layers have electricalproperties that prevent high voltage drops across the belt, that preventhigh pre-nip transfer fields via lateral conduction of the belt, thatavoid charge buildup, and that prevent high current flow.

While the electrical properties of a seamed intermediate transfer beltshould be controlled so as to integrate that belt with otherelectrophotographic printer subsystems, acceptable belt resistivitiesshould be typically less than 1×10¹³ ohm-cm volume resistivity and morethan 1×10⁸ ohms/square lateral resistivity. Lateral resistivity isdefined as being the volume resistivity in the direction of belt motiondivided by the layer's thickness. In some cases the belt resistivity issensitive to the applied field. In such cases the volume resistivityshould be referenced to a corresponding range of applied fields. Whilethe applied field depends on the particular system design, the upperlimit volume resistivity is generally measured at a field correspondingto between 10 to 100 volts across the layer thickness, and the lowerlimit lateral resistivity of interest is generally measured between 500to 2000 volts/cm.

Seamed intermediate transfer belts can also have constraints on thelower limit of their volume resistivity in the thickness direction.Typically such constraints occur in systems where the intermediate beltcontacts or moves so close to a low resistivity surface in a transferzone that the possibility of high resistive or corona discharge currentdensity flow between the belt and the low resistivity surface exists.One example of such a system is a drum photoreceptor that has scratchesor pin holes in an otherwise insulating drum coating. An intermediatetransfer belt can momentarily come very close or even touch the highlyconductive drum substrate at the scratches or pin holes in the transferzone. Another example is a system that transfers toner from oneintermediate transfer belt to a second, relatively conductiveintermediate transfer receiver. In such systems if the intermediatesystem composite resistance, R_(comp), in the transfer nip is too low,problems can occur due to undesirably high local current density flowbetween the intermediate transfer belt surface and the low resistivitycontacting surfaces in the transfer nip. Problems can include local“shorting” between the intermediate transfer belt surface and thereceiver that can cause momentary loss of the local appliedelectrostatic transfer field, and thereby result in degraded tonertransfer. The composite resistance, R_(comp), in the transfer nip is thesum of all possible “shorting” resistance paths in the transfer nips.The composite resistance path includes, for example, the effectiveresistance path of the transfer field generating device, the resistancepath of the intermediate belt substrate, and the resistance path of theintermediate belt overcoat.

Shorting issues can be solved by insuring that there is a “sufficientlyhigh” composite resistance path within the transfer nips. Whether acomposite resistance is “sufficiently high” depends on the system, andespecially on the type of power supply used for the field generatingsystem. The shorting issue occurs when the shorting leakage current flowin the intermediate transfer nips is “too high.” The shorting leakagecurrent flow is the applied potential difference in the transfer nipdivided by the composite resistance. For example, the current will be“too high” when it exceeds the power supply current capability. Typicalpower supplies used in transfer systems limit the current to less than 2milliamps, so such shorting currents are “too high” for most systems.Other power supplies used in transfer systems use constant current powersupply control. In such systems, the applied transfer fields are relatedto the portion of the controlled current that is not shorting leakagecurrent. Thus any shorting leakage current tends to significantly reducethe transfer fields. Typically, with a constant current control, theshorting leakage current will be “too high” when the leakage currentexceeds about 20% of the nominal constant current control.

The allowed lower resistivity limit of an intermediate transfer beltalso depends on other system inputs. For example, the shorting problemcaused by photoreceptor defects depends on the size of the defects thatare present in the system. So, in systems that maintain very good defectfree high dielectric strength drum coating layers, shorting to drumdefects can be avoided even with extremely low volume resistivityintermediate transfer belts. Thus the allowed lower limit for the volumeresistivity can vary widely. Still, experience suggests guidelines toavoid shorting problems. To avoid problems in systems that have a “smallarea shorting contact” in the transfer nip, such as in the drum defectexample, the volume resistivity of the topmost layer on the intermediatetransfer belt should be above 10⁷ ohm-cm, with a preference of beingabove 10⁸ ohm-cm. The resistivity values apply for intermediate materiallayer thickness that is at least around 25 microns thick or larger. Ifthe resistivity of the materials used for the intermediate transfer beltare sensitive to the applied field, the volume resistivity should bemeasured with an applied potential difference across the transfer beltthat is similar to the applied potential difference used in the transfersystem. With low resistivity intermediate materials, this is typicallyaround 200 to 1000 volts across the thickness of the intermediate beltmaterial.

It can be appreciated by those skilled in the art of electrostatictransfer that the electrical properties allowed for any particularintermediate transfer belt application can depend on many factors. Thussome systems can achieve acceptable intermediate transfer performancewith intermediate transfer belt material layers having a much higherresistivity than 1×10¹³ ohm-cm and with materials layers having a muchlower lateral resistivity than 1×10⁸ ohms/square. For example, a problemwith very high resistivity intermediate materials layers is chargebuildup between transfer stations or belt cycling. However, chargebuildup problems can be minimized with belt material layers having muchhigher resistivity than 1×10¹³ ohm-cm if suitable charge conditioningdevices such as corotrons or scorotrons are provided along thecircumference of the intermediate transfer belt configuration to reduceand level the unwanted charge buildup. Generally, with very highresistivity intermediate material layers in color systems, chargeconditioning devices are necessary but not sufficient. To be fullyeffective the total dielectric thickness of any very high resistivitybelt layers must also be kept low, typically less than 25 microns, andpreferably less than 10 microns. Unwanted cost and complexity isintroduced by the need for cyclic charge conditioning devices, andtherefore intermediate systems most typically prefer suitably lowerresistivity intermediate materials.

Similarly, although not preferred, some systems can use intermediatetransfer belts that have material layers on the belt that have lateralresistivity less than 1×10⁸ ohms/square. Such belts are typically notdesired because, if any layer of an intermediate transfer belt has alateral resistivity somewhat less than 1×10⁸ ohms/square, highelectrostatic transfer fields can occur in the pre-nip region of thetransfer zones before contact of the belt with the toner. High pre-nipfields can cause toner transfer across large air gaps in the pre-nipregion and this can result in undesirable toner disturbance or splatterof the toner beyond the edges of the image. Also, due to lateralconduction of charge away from the contact transfer nip, any increase inthe transfer fields in the contact nip automatically increases thefields in the pre-nip region. This can cause pre-nip air breakdownbetween the toner and intermediate belt prior to the contact nip. Chargeexchange due to pre-nip air breakdown limits the applied transfer fieldsand it tends to reverse the polarity of any untransferred toner in thepre-nip region. This can then limit transfer efficiency and it can causeimage defects due to the nonuniform nature of typical pre-nip airbreakdown. However, if the toner adhesion in a particular system is lowsuch that the required electrostatic transfer fields in the nip for goodtransfer are low, pre-nip field problems caused by lateral conductioncan be a small issue. Then, some systems can achieve acceptable transferperformance in spite of having low intermediate belt lateralresistivity.

A complication in enabling transfer of toner onto and off of a seamedintermediate transfer belt is that the electrical properties of anintermediate transfer belt and the seam are generally not constant. Forexample, the resistivity of most materials used for seamed intermediatetransfer belts depend on the fields within the material. Thoseelectrical properties can also depend on the environment, aging, anduse. In addition, many manufacturing processes can produce a relativelywide distribution of resistivity values for film materials due to smallvariations in the resistivity control factors in the manufacturingprocess. Thus, the materials used for intermediate transfer belts andfor the seam adhesives can have resistivities that vary by more than afactor of 100. Therefore, a transfer system in which toner istransferred onto and off of a seamed intermediate transfer belt must bedesigned to operate over a wide range of electrical properties.

One method of compensating for the wide variations of the electricalproperties of intermediate transfer belts is to use a “set pointcontrol” approach. For example, a transfer setpoint, such as an appliedvoltage or field-generating device, can be adjusted to compensate forenvironmental effects such as temperature and relative humidity thatwould otherwise change the intermediate transfer belt's electricalproperties. Such an approach is effective because the electricalproperty changes due to the environment are substantially the same atall points along the belt. In general, the “set point” control approachenables a wider tolerance in the electrical properties of theintermediate transfer belt, provided those properties do not greatlyvary along the belt's periphery. However, the set point control approachloses effectiveness when the electrical properties of the intermediatetransfer belt vary over small distances, such as across a seam gap.Therefore, a seamed intermediate transfer belt suitable for receivingand transferring toner onto and off of its seam would generally requireseam electrical properties that maintain a close relationship to thechanging electrical properties of the rest of the belt. This presents aproblem because the electrical properties of many otherwise good seamadhesives may not have the same responses as the rest of the belt.

While the difficulties of transferring toner onto and off of anintermediate transfer belts are numerous, so are the advantages.Therefore, an electrophotographic marking machine having an imageableseam intermediate transfer belt would be beneficial.

SUMMARY OF THE INVENTION

The principles of the present invention provide for electrophotographicmarking machines having imageable seam intermediate transfer belts. Anelectrophotographic marking machine according to the principles of thepresent invention includes a charged photoreceptor, an exposure stationfor producing an electrostatic latent image on the chargedphotoreceptor, a developer for depositing toner onto the electrostaticlatent image to produce a toner image, and an intermediate transferstation for receiving the toner image. The intermediate transfer stationincludes an imageable seam intermediate transfer belt for receiving thetoner image such that the toner image spans the seam. The intermediatetransfer station is further for transferring the toner image onto asubstrate. An electrophotographic marking machine according to theprinciples of the present invention beneficially further includes afusing station for fusing the toner images with the substrate and acleaning station for removing excess toner and debris from thephotoreceptor.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features of the present invention will become apparent as thefollowing description proceeds and upon reference to the drawings, inwhich:

FIG. 1 is an isometric representation of a puzzle cut seamed substratelayer;

FIG. 2 shows a puzzle cut tab pattern used in the substrate layer ofFIG. 1;

FIG. 3 illustrates the puzzle cut tabs of FIG. 2 interlocked together;

FIG. 4 illustrates the puzzle cut tabs of FIG. 3 with the kerf of FIG. 3filled with an adhesive;

FIG. 5 is a cut-away view of an intermediate transfer belt in which anadhesive is applied over a substrate layer to form an outer coating;

FIG. 6 is a cut-away view of an intermediate transfer belt in which anadhesive is applied to the seam and a coating is added to over thesubstrate layer and adhesive to form an overcoating;

FIG. 7 is a close up side view of an intermediate transfer belt in whichan adhesive is applied to overlapping ends of a substrate;

FIG. 8 is a schematic depiction of an imageable seam intermediatetransfer belt in a transfer nip;

FIG. 9 is a schematic depiction of a color electrophotographic markingmachine having an imageable seam intermediate transfer belt; and

FIG. 10 is an enlarged schematic depiction of a fusing station used inthe color electrophotographic marking machine of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

While the principles of the present invention are described below inconnection with an embodiment of an electrophotographic marking machine,it should be understood that the present invention is not limited tothat embodiment. On the contrary, the present invention is intended tocover all alternatives, modifications, and equivalents as may beincluded within the spirit and scope of the appended claims.

An electrophotographic marking machine according to the principles ofthe present invention includes a seamed intermediate transfer belt thatreceives toner over the seam region and that subsequently transfers thattoner onto a receiver. Such an intermediate transfer belt issubsequently referred to as an imageable seam intermediate transferbelt. Because of the importance of the imageable seam intermediatetransfer belt the construction of that belt will be described first.Such a belt begins with a substrate layer 10 as shown in FIG. 1. Inpractice, that substrate layer is usually semiconductive. The substratehas its ends joined together to form a continuous member using anexternally applied adhesive. Alternatively, the continuous member couldbe formed by melting the substrate's ends together using heat welding,solvent welding, or other joining methods. The region around the joinedends can have properties that are significantly different than theregions far from the seam region of the joined belt. These localadjacent regions will be referred to as the “gap” region.

Beneficially the ends are joined using mechanically interlocking “puzzlecut” tabs that form a seam 11. While the seam is illustrated as beingperpendicular to the two parallel sides of the substrate layer the seamcould be angled or slanted with respect to the parallel sides. While theseam 11 is puzzle cut it could also be formed in other fashions, such asusing an overlapping seam (see FIG. 7). However, the puzzle cut iscurrently the preferred case. Reference U.S. Pat. Nos. 5,487,707;5,514,436; 5,549,193; and 5,721,032 for additional information on puzzlecut patterns. Typically the seam 11 is about ¼ inch wide.

The substrate layer 10 can be made from a number of different materials,including polyesters, polyurethanes, polyimides, polyvinyl chlorides,polyolefins (such as polyethylene and polypropylene) and/or polyamides(such as nylon, polycarbonates, or acrylics). If required, the selectedmaterial is modified by the addition of an appropriate filler such thatthe substrate layer has a desired electrical conductivity. Appropriatefillers can include for example carbon, Accuflor carbon, and/orpolyanaline. The substrate layer material should have the physicalcharacteristics appropriate to an intermediate transfer application,including good tensile strength (Young's modulus, typically 1×10³ to1×10⁶ newtons/M², resistivity (typically less than 10¹³ ohm cm volumeresistivity, greater than 10⁸ ohms/square lateral resistivity), thermalconductivity, thermal stability, flex strength, and high temperaturelongevity. More information regarding electrical conductivity is givensubsequently.

FIG. 2 illustrates a puzzle cut tab pattern. Each tab is comprised of aneck 14 and a node 16 that fit into female 15 interlocking portions. Thetabs can be formed using any conventional shaping technique, such as diecutting, laser cutting, or cutting wheel. The interlocking tab matingsfit to reduce the stress concentration between the interlocking elementsand to permit easy travel around curved members, such as the rollers 12shown in FIG. 1. While FIG. 2 shows one puzzle cut pattern, others arepossible. Reference U.S. Pat. No. 5,997,974, issued Dec. 7, 1999,entitled “Invisible Seam Electrostatographic Belt” for other puzzle cutpatterns.

FIG. 3 illustrates the puzzle cut tabs of FIG. 2 interlocked together.Physically interlocking the puzzle cut tabs may require pressure whenmating the tabs. Interlocking produces a void between the mutuallymating elements that is called a kerf 20. As shown in FIG. 4 theinterlocking tabs are held together using an adhesive 22 that files thekerf. The adhesive is designed to be physically, chemically, thermally,mechanically, and electrically compatible with the substrate layermaterial. Seams with a 25μ kerf have been typical for the puzzle cutseam while a kerf less than about 5μ can be preferred.

To be compatible with the substrate layer material the adhesive shouldproduce a seam that is strong, smooth, and mechanically uniform. Themechanical strength and flexibility of the seam should be such that thebelt operates satisfactorily for at least 100,000 cycles but preferablymore than 1,000,000 cycles. Furthermore, topography parameters such asthe height differential between the seamed and the unseamed portions ofthe substrate layer and the peak to valley distortions of the top andbottom of the seam need to be below critical levels. Acceptabletopography parameter levels can depend on system factors, on theelectrical properties of the adhesive, and on whether or not coatingsare applied to the intermediate belt after the seaming, all of whichwill be discussed. However, the seam should typically be substantiallyfree of large “bumps,” “valleys,” and other short-wavelengthdistortions.

In practice the adhesive 22 should have a viscosity such that it readilywicks into the kerf. Additionally, the surface energy of the adhesiveshould be compatible with the substrate layer material such that theadhesive adequately wets and spreads in the kerf. Furthermore, theadhesive should remain flexible and should adhere well to the substratelayer material. Finally, the adhesive also should have low shrinkageduring curing. Appropriate manufacturing practices should be used toprevent excessive long-wavelength and short-wavelength disturbances. Asan example, the adhesive can be a hot melt adhesive that is heated andpressed into the seam such that the adhesive is flattened, making it asmechanically uniform as possible with the substrate layer 10.Alternatively, the adhesive can be an epoxy-like material, a UV curableadhesives including acrylic epoxies, polyvinyl butyrals, or the like.Further, the “adhesive” can be substantially the substrate materialitself, either applied during a separate adhesive application step orelse by melting the two ends sufficiently to cause adhesion of themutually mating elements. Following the application of the adhesive theseam 11 can be finished by buffing, sanding, or micro polishing toachieve a smooth topography.

Achieving a smooth topography is important for an imageable seamintermediate transfer belt. As previously discussed, sufficiently smoothshort-wavelength and long-wavelength topographies are required on thetoner-bearing side to avoid transfer and cleaning issues. A sufficientlysmooth topography is also needed on the back-side of to avoid transferissues. The short-wavelength seam disturbances for the back-side of thebelt should be less than 10 microns to avoid transfer problems. On thetoner-bearing side the short-wavelength seam disturbances should be lessthan 10 microns, more preferably less than about 5 microns, and mostpreferably less than 1 micro. If a blade cleaning system is used asmoother toner-bearing surface, say better than 1 micronshort-wavelength seam disturbances, is better. In summary, the seamtopography for an imageable seam intermediate transfer belt shouldpreferably be substantially the same as the belt topography in regionsaway from the seam. However, some degree of degraded seam topography canbe tolerated as long as the seam topography is within the constraintsallowed for the particular application. Thus, while the previouslydiscussed seam topography parameters are reasonable guidelines, theactual parameters are best determined experimentally for a particularapplication.

The relative electrical properties of the adhesive and the substrate arevery important because they significantly affect the transfercharacteristics of the resulting seam as compared to the transfercharacteristics of the rest of the belt. Therefore, the adhesive shouldproduce a seam that has electrical properties that corresponds to thatof the substrate layer. That is, under operating conditions a seamshould create an electrostatic transfer field in the toner transferzones that is within at least 20%, preferably within 10%, of theelectrostatic transfer field that is present for the remainder of thebelt. Ideally the seam electrical properties are substantially the sameas the substrate layer and have substantially the same electricalproperty dependence as the substrate on all important factors, suchenvironment, applied field, and aging. However, significant differencesin electrical properties can be allowed for some imageable seamconditions as discussed subsequently. The adhesive electrical propertiescan be met by mixing fillers or additives with an adhesive. For example,an adhesive might contain silver, indium tin oxide, CuI, SnO2, TCNQ,Quinoline, carbon black, NiO and/or ionic complexes such as quaternaryammonium salts, metal oxides, graphite, or like conductive fillers.

With the adhesive in the puzzle cut seam, one or more overcoats areapplied using conventional processes such as dip coating, flow coatingand spray coating. As shown in FIG. 5, an imageable seam intermediatetransfer belt 40 might have an overcoat 38 that is comprised of theadhesive 22 itself. However, because intermediate transfer belts havestringent toner release requirements it may be desirable to use aspecial release overcoat 42 on the substrate layer 10 and on theadhesive 22 as shown in cut-away in FIG. 6. Additional coating layer(s)can be advantageous for various reasons. An overcoat can reinforce theseam strength. They can also reduce the electrostatic transfer fieldperturbations caused by a mismatch between the electrical properties ofthe adhesive and the substrate. Overcoats can also insure that thefriction and toner release properties in the seam region are the same asover the rest of the belt. This increases the range of acceptableadhesives and prevents cleaning and transfer differences that mightotherwise occur. Finally, overcoats can smooth out the seam region andthus reduce seam topography problems. However, overcoats increase thecost and complexity of manufacturing an imageable seam intermediatetransfer belt.

While the foregoing has described the use of puzzle-cut tabs, theprinciples of the present invention can be practiced with other types ofjoints. For example, FIG. 7 illustrates a cut-way view of anintermediate transfer belt 60. That belt includes a substrate layer 62having ends 64 and 66 that overlap. Between the overlap, and extendingover the top and bottom of the substrate layer, is an adhesive 68.Beneficially the adhesive tapers away from the overlap area such that asmooth transition is made. A smooth transition is needed to avoidpreviously discussed topography problems, and it also improves themechanical characteristics of the intermediate transfer belt 60 when itpasses over a roller. Over the top side of the belt is an overcoat 70.

The overcoats discussed with reference to FIGS. 5-7 beneficially havelow friction and good toner release characteristics for enabling goodtransfer and cleaning. A friction coefficient less than about 1.0, andpreferably less than 0.5, is suitable. Preferred overcoat materialsinclude low surface free energy materials such as TEFLON™ typefluoropolymers, including fluorinated ethylene propylene copolymer(FEP), polytetrafluoroethylene (PTFE), polyfluoroalkoxypolytetrafluoroethylene (PFA TEFLON™); fluoroelastomers such as thosesold by DuPont under the tradename VITON™; and silicone materials suchas fluorosilicones and silicone rubbers.

Referring now back to FIG. 6, in a preferred embodiment intermediatetransfer belt 41 the substrate layer 10, the adhesive 22, and theovercoat 42 are all semiconductive. An electrophotographic markingmachine 100 that makes beneficial use of that belt is illustrated inFIGS. 9 and 10. With reference those figures the electrostatographicprinter 100 includes the intermediate transfer belt 41, which is drivenover guide rollers 114, 116, 118, and 120. The intermediate transferbelt moves in a process direction shown by the arrow A. For purposes ofdiscussion, the intermediate transfer belt 41 includes sections thatwill be referred to as toner areas. A toner area is that part of theintermediate transfer belt that receives actions from the variousstations positioned around the intermediate transfer belt. While theintermediate transfer belt 41 may have multiple toner areas, each tonerarea is processed in the same way.

A toner area is moved past a set of four toner image stations 122, 124,126, and 128. Each toner image station operates to place a unique colortoner image on the toner image of the intermediate transfer member 41.Each toner image producing station operates in the same manner to formdeveloped toner image for transfer to the intermediate transfer member.

While the image producing stations 122, 124, 126, 128 are described interms of photoreceptive systems, they may also be ionographic systems orother marking systems that form developed toner images. Each toner imageproducing station 122, 124, 126, 128 has an image bearing member 130.The image bearing member 130 is a drum supporting a photoreceptor 121(see FIG. 10).

Turn now to FIG. 10, which shows an exemplary toner image producingstation. That image bearing station generically represents each of thetoner image producing station 122, 124, 126, 128. As shown, thephotoreceptor 121 is uniformly charged at a charging station 132. Thecharging station is of well-known construction, having charge generationdevices such as corotrons or scorotrons for distribution of an evencharge on the surface of the image bearing member. An exposure station134 exposes the charged photoreceptor 121 in an image-wise fashion toform an electrostatic latent image on an image area. The image area isthat part of the image bearing member which receives the variousprocesses by the stations positioned around the image bearing member130. The image bearing member may have multiple image areas; however,each image area is processed in the same way.

The exposure station 134 preferably has a laser emitting a modulatedlaser beam. The exposure station then raster scans the modulated laserbeam onto the charged image area. The exposure station 134 canalternately employ LED arrays or other arrangements known in the art togenerate a light image representation that is projected onto the imagearea of the photoreceptor 121. The exposure station 134 exposes a lightimage representation of one color component of a composite color imageonto the image area to form a first electrostatic latent image. Each ofthe toner image producing stations 122, 124, 126, 128 will form anelectrostatic latent image corresponding to a particular color componentof a composite color image.

The exposed image area is then advanced to a development station 136.The developer station 136 has a developer corresponding to the colorcomponent of the composite color image. Typically, therefore, individualtoner image producing stations 122, 124, 126, and 128 will individuallydevelop the cyan, magenta, yellow, and black that make up a typicalcomposite color image. Additional toner image producing stations can beprovided for additional or alternate colors including highlight colorsor other custom colors. Therefore, each of the toner image producingstations 122, 124, 126, 128 develops a component toner image fortransfer to the toner area of the intermediate transfer member 41. Thedeveloper station 136 preferably develops the latent image with acharged dry toner powder to form the developed component toner image.The developer can employ a magnetic toner brush or other well-knowndevelopment arrangements.

The image area having the component toner image then advances to thepretransfer station 138. The pretransfer station 138 preferably has apretransfer charging device to charge the component toner image and toachieve some leveling of the surface voltage above the photoreceptor 121to improve transfer of the component image from the image bearing member130 to the intermediate transfer member 112. Alternatively thepretransfer station 138 can use a pretransfer light to level the surfacevoltage above the photoreceptor 121. Furthermore, this can be used incooperation with a pretransfer charging device.

The image area then advances to a transfer nip 140 defined between theimage bearing member 130 and the intermediate transfer member 41. Theimage bearing member 130 and intermediate transfer member 41 aresynchronized such that each has substantially the same linear velocityat the first transfer nip 140. The component toner image is thenelectrostatically transferred from the image bearing member 130 to theintermediate transfer member 41 by use of a field generation station142. The field generation station 142 is preferably a bias roller thatis electrically biased to create sufficient electrostatic fields of apolarity opposite that of the component toner image to thereby transferthe component toner image to the intermediate transfer member 41.Alternatively the field generation station can be a corona device orother various types of field generation systems known in the art. Aprenip transfer blade 144 mechanically biases the intermediate transfermember 41 against the image bearing member 130 for improved transfer ofthe component toner image.

After transfer of the component toner image, the image bearing member130 then continues to move the image area past a preclean station 139.The preclean station employs a pre clean corotron to condition the tonercharge and the charge of the photoreceptor 121 to enable improvedcleaning of the image area. The image area then further advances to acleaning station 141. The cleaning station 141 removes the residualtoner or debris from the image area. The cleaning station 141 preferablyhas blades to wipe the residual toner particles from the image area.Alternately the cleaning station 141 can employ an electrostatic brushcleaner or other well-know cleaning systems. The operation of thecleaning station 141 completes the toner image production for each ofthe toner image producing stations.

Turning back to FIG. 9, the individual toner image producing stations122, 124, 126, and 128 each transfer their toner images onto theintermediate transfer member 41. A first component toner image isadvanced onto the intermediate transfer member 41 at the transfer nip140 of the image producing station 122. Prior to the toner area arrivingat the transfer nip of the image producing station 122 the toner area isuniformly charged by a conditioning station 146. This reduces the impactof any stray, low or oppositely charged toner that might result in backtransfer of toner into the image producing station 122. Such aconditioning station is positioned before each transfer nip 140.

The geometry of the interface of the intermediate transfer member 41with an image bearing member 130 has an important role in assuring goodtransfer of the component toner image. The intermediate transfer member41 should intimately contact each image bearing member 130. To that endthe pre-nip pressure blade 144 creates an intimate pre-nip contact.Then, to bring about toner transfer, the field generation station 142,such as a corotron or a bias charging roller, produces an electrostaticbias on the intermediate transfer member 41 such that the toner image oneach image bearing member 130 is transferred onto the intermediatetransfer member 41. Those toner images are transferred in so that theimages are registered. That is, each of the individual color componentimages on the toner image producing stations 122, 124, 126, and 128 aretransferred onto the intermediate transfer member 41 such that the humaneyes perceives a desired composite color image.

The intermediate transfer member 41 then transports the composite tonerimage to a pre-transfer charge conditioning station 152 that levels thecharges at the toner area of the intermediate transfer member 41 andprepares them for transfer to a transfuse member 150. The pre-transfercharge conditioning station 152 is preferably a scorotron. A secondtransfer nip 148 is defined between the intermediate transfer member 41and the transfuse member 150. A field generation station 142 and a pre-transfer nip blade 144 engage the intermediate transfer member 41 andperform similar functions as the field generation stations andpre-transfer blades 144 adjacent the transfer nips 140. The compositetoner image is then transferred electrostatically onto the transfusemember 150.

The transfer of the composite toner image at the second transfer nip 148can be heat assisted if the temperature of the transfuse member 150 ismaintained at a sufficiently high optimized level and the temperature ofthe intermediate transfer member 41 is maintained at a considerablylower optimized level prior to the second transfer nip 148. Themechanism for heat assisted transfer is thought to be softening of thecomposite toner image during the dwell time of contact of the toner inthe second transfer nip 148.

This composite toner softening results in increased adhesion of thecomposite toner image toward the transfuse member 150 at the interfacebetween the composite toner image and the transfuse member. This alsoresults in increased cohesion of the layered toner pile of the compositetoner image. The temperature on the intermediate transfer member 41prior to the second transfer nip 148 needs to be sufficiently low toavoid too high a toner softening and too high a resultant adhesion ofthe toner to the intermediate transfer member 41. The temperature of thetransfuse member 150 should be considerably higher than the tonersoftening point prior to the second transfer nip to insure optimum heatassist in the second transfer nip 148. Further, the temperature of theintermediate transfer member 41 just prior to the second transfer nip148 should be considerably lower than the temperature of the transfusemember 150 for optimum transfer in the second transfer nip 148.

Turning now to FIG. 10, the transfuse member 150 is guided in a cyclicalpath by guide rollers 174, 176, 178, 180. Guide rollers 174, 176 aloneor together are preferably heated to thereby heat the transfuse member150. The intermediate transfer member 41 and transfuse member 150 arepreferably synchronized to have the generally same velocity in thetransfer nip 148. Additional heating of the transfuse member is providedby a heating station 182. The heating station 182 is preferably formedof infra-red lamps positioned internally to the path defined by thetransfuse member 150. The transfuse member 150 and a pressure roller 184define a third transfer nip 186 therebetween.

A releasing agent applicator 188 applies a controlled quantity of areleasing material, such as a silicone oil to the surface of thetransfuse member 150. The releasing agent serves to assist in release ofthe composite toner image from the transfuse member 150 in the thirdtransfer nip 186.

The transfuse member 150 preferably has a top most layer formed of amaterial having a low surface energy, for example silicone elastomer,fluoroelastomers such as Viton™, polytetrafluoroethylene,perfluoralkane, and other fluorinated polymers. The transfuse member 150will preferably have intermediate layers between the top most and backlayers constructed of a Viton™ or silicone with carbon or otherconductivity enhancing additives to achieve the desired electricalproperties. The back layer is preferably a fabric modified to have thedesired electrical properties. Alternatively the back layer can be ametal such as stainless steel.

A substrate 170 is then advanced toward the third transfer nip 186. Thesubstrate 170 is transported and registered by a material feed andregistration system 169 into a substrate pre-heater 173. The substratepre-heater 173 includes a transport belt that moves the substrate 170over a heated platen. The heated substrate 170 is then directed into thethird transfer nip 186.

At the third transfer nip the composite toner image is transferred andfused to the substrate 170 by heat and pressure to form a completeddocument 172. The document 172 is then directed into a sheet stacker orother well know document handing system (not shown).

A cooling station 166 cools the intermediate transfer member 41 afterthe second transfer nip 148. A cleaning station 154 engages theintermediate transfer member 41 and removes oil, toner or debris thatmay be remain onto the intermediate transfer member 41. The cleaningstation 154 is preferably a cleaning blade alone or in combination withan electrostatic brush cleaner, or a cleaning web.

While the foregoing is sufficient to understand the general operation ofelectrophotographic printing machines that use imageable seamintermediate transfer belts, in practice how to implement suchelectrophotographic printing machines is not obvious. This is becauseimageable seam intermediate transfer belts that produce acceptable finalimages, such as the intermediate transfer belt 41, are subject tonumerous electrical and mechanical constraints, limitations, and designproblems. A discussion of some of those factors follow. It should benoted that while the intermediate transfer belt 41 includes anovercoating, it is possible to construct imageable seam intermediatetransfer belts that do not use overcoats. However, the inclusion of anovercoat is usually preferable as it simplifies the overall design ofthe electrophotographic printing machine. Reference U.S. patentapplication Ser. No. 09/460,896, entitled “Imageable Seam IntermediateTransfer Belt Having An Overcoat” now U.S. Pat. No. 6,245,402 and U.S.patent application Ser. No. 09/460,821, entitled “Imageable SeamIntermediate Transfer Belt,” both filed on Dec. 14, 1999.

In intermediate transfer systems, significant charge is deposited ontothe intermediate transfer belt when passing through a transfer zone. Ifthe overcoat resistivity is too high, the voltage drop across theovercoat will build up after each successive pass through the transferzone. This can adversely interfere with transfer performance. Asufficiently low resistivity overcoat can dissipate the voltage dropacross the overcoat thickness via conduction during the dwell timebetween successive pass though toner transfer zones. The preferredresistivity ρ_(C) for this desired charge dissipation depends on a“cyclic charge relation time.” The cyclic charge relaxation timeT_(ρcyC), preferably should be less than a characteristic “cyclic dwelltime”, T_(dcy), that being the time that a section of the intermediatetransfer belt takes to travel between successive transfer zones. Thecyclic dwell time is the distance between successive transfers dividedby the belt speed.

If the overcoat resistivity is independent of the applied field,exponential charge decay across the overcoat thickness will occur andT_(ρcyC) is given by: T_(ρcyC) =K_(C)ρ_(C)ε₀, where K_(C) is thedielectric constant of the overcoat, ρ_(C) is the volume resistivity ofthe overcoat thickness, and ε₀ is the permitivity of air. If theovercoat resistivity changes with the applied field a simple exponentialcharge decay will not occur. However, as an approximation thecharacteristic cyclic charge relaxation time expressionT_(ρcyC)=K_(C)ρ_(C)ε₀, can still be useful if the overcoat resistivityis specified at an applied field of interest that prevents too large ofa voltage drop across the thickness. For overcoat materials that have afield sensitive resistivity, the overcoat resistivity that should beused in the cyclic charge relaxation expression preferably should bethat determined at an applied field corresponding to less than 100 voltsand more preferably less than 10 volts across the overcoat thickness.Sufficiently low resistivity at such fields will insure that there willbe a low voltage drop across the overcoat. As an example, if an overcoathas K_(C)=3, an intermediate transfer system having a process speed near10 in/sec and a distance between successive transfers of around 10inches, the overcoat resistivity for charge dissipation preferablyshould be around ρ_(C)<3.8×10¹² ohm-cm. For a different process speednear 3 in/sec and otherwise similar conditions, the overcoat resistivityfor charge dissipation preferably should be around ρ_(C)<10¹³ ohm-cm.Overcoat resistivities near the upper range of the high resistivitylimits are mainly acceptable when the coating dielectric thickness,D_(C), is sufficiently small, preferably smaller than around 25 micons.The dielectric thickness of the overcoat is the actual overcoatthickness divided by the dielectric constant of the overcoat K_(C). Asdiscussed below, thick overcoats can introduce additional transferconcerns, and thick overcoats tend to work better with a lowerresistivity than the upper limits discussed thus far.

A sufficiently thick overcoat, for example an thickness that is at leastcomparable to about half of the kerf gap and is preferably somewhatgreater than the seam kerf gap size, can enable some imageable seamconditions that would otherwise not be imageable. As discussed furtherlater, increased coating thickness tends to “hide” the effect ofotherwise unacceptable adhesive electrical properties. This is becausethe perturbing effect on the electrostatic fields of seam gap electricalproperties tends to get smaller with distance from the seam gap. Thedetails of this will be discussed later. For now, note there can be aninterest in using thick coatings for enabling certain optional imageableseam conditions.

There can be further preferred overcoat resistivity ranges if thedielectric thickness of the overcoat, D_(C), is large, for exampletypically if D_(C) is near or somewhat larger than around 25 micons. Ifthe resistivity of the overcoat is above a critical value, the overcoatwill begin to behave similar to an “insulator” during the dwell timenear the transfer nips. Then, as is well known in the art ofelectrostatics, the voltage drop across the overcoat in the transfer nipwill increase with increasing overcoat dielectric thickness. So, toachieve the same transfer field acting on the toner, the appliedvoltages on the transfer field generation device will have to increaseas the overcoat dielectric thickness increases to compensate for thehigher voltage drop across the overcoat. High voltages on transfer fieldgeneration devices are not desired because they can stress the systemrelative to causing unwanted higher fields in the pre-nip region of thetransfer nip, they tend to add cost to the power supply, and in extremestoo high a voltage can lead to undesired constraints on clearancedistances needed to avoid arcing problems. So, if the overcoatdielectric thickness is too high when the resistivity of the overcoat isalso too high, the applied voltages can be higher than desired. If theresistivity of the overcoat is less than a critical value, chargeconduction through the overcoat thickness during the transfer nip dwelltime reduces the voltage drop across the overcoat during the transfernip dwell time. Thus the use of a sufficiently low overcoat resistivitycan prevent the problem of undesirably large transfer voltages in spiteof relatively large overcoat dielectric thickness.

The condition for a sufficiently low overcoat resistivity can beestimated by the condition that a characteristic “nip charge relaxationtime” for charge flow through the overcoat thickness in the transfernip, T_(ρnip C), is at least comparable and is preferably smaller than acharacteristic effective “nip dwell time” that a section of theintermediate belt spends in and very near the contact nip of thetransfer field generation device, T_(dnip),. The nip dwell time T_(dnip)can typically be estimated as the effective nip width W in the processtravel direction of the field region near the bias field generationdevice in the transfer nip where the fields are building up, divided bythe speed of the intermediate belt. For a bias roller field generationdevice, the effective nip width W is estimated as the size of the rollercontact nip width plus the widths in the pre and post nip regions wherethe pre and post nip air gaps are around 50 micorns. For a simple coronageneration device, the effective nip width W is estimated as the widthof the corona current density beam profile. For a corotron system theparameter T_(ρnipC) is estimated from: T_(ρnip)=K_(C)ρ_(C)ε₀. For a biasroller system the parameter T_(ρnipC) is estimated from:T_(ρnip)=K_(C)ρ_(C)ε₀[1+D_(C)/ΣD_(I)], where ΣD_(I) is the sum of thedielectric thickness of the toner, air, and other insulating layers,other than the overcoat within the transfer nip. For overcoats having afield dependent resistivity, the overcoat resistivity used in thisestimate should typically be determined at a field corresponding to lessthan 100 volts, and more preferably around 10 volts, across thethickness of the overcoat. As an example, with a bias roller a typicaleffective nip width is around 0.1 inches and the parameter ΣD_(I) istypically around 20 microns. For example, at a process speed of 10in/sec and with a overcoat having a dielectric constant K=3, a desiredresistivity to prevent high voltage drop across a 150 micron thickcoating is around ≦1×10¹⁰ ohm-cm. As another example, for a 25 micronthick overcoat and otherwise similar parameters to the previous example,a desired resistivity to prevent significant voltage drop across theovercoat during the transfer dwell time is around ≦3×10¹⁰ ohm-cm. Forthis last example, if the process speed is 3 in/sec, a desired overcoatresistivity to prevent significant voltage drop across the overcoatduring the transfer nip dwell time is around ≦10¹¹ ohm-cm. With anovercoat having a “nip charge relaxation time” smaller than acharacteristic effective “nip dwell time”, there are minimal constraintson the thickness of the overcoat. From the examples, if a moderatelyhigh dielectric thickness overcoat is used most systems will typicallyprefer overcoat resistivity less than around 10¹¹ ohm-cm and morepreferably will typically prefer overcoat resistivity less than around10¹⁰ ohm-cm if a very high dielectric thickness overcoat is used.

In the above discussions and in various other discussions of electricalproperties in this patent, resistivities are referenced. However,typically a more fundamental characteristic is the “charge relaxationtimes.” Charge relaxation times can be directly measured in a systemusing known techniques in the art of electrostatics, and chargerelaxation times can be a more preferred way of specifying the suitableelectrical properties for imageable seam intermediate transfer belts.

The above defined resistivity range where the “nip charge relaxationtime” is smaller than the characteristic effective “nip dwell time” isalso a desirable electrical property of a seam adhesive when significantadhesive overspill onto the substrate layer occurs. The expressionspreviously given for the nip relaxation time estimates are the same forthe adhesive overspill if the resistivity P_(OA), dielectric thicknessDO_(A) and dielectric constant K_(OA) of the overspill are used insteadof the resistivity and dielectric constant of the overcoat.

To understand the undesirable effects of a high resistivity adhesiveoverspill, refer back to FIG. 5, which shows adhesive overspill on thebackside of a belt.

The adhesive overspill adds an extra adhesive thickness in the seamregion that is not present away from the seam. If the adhesiveresistivity ρ_(OA) is too high the adhesive acts like an “insulator”during the characteristic dwell time spent in the transfer fieldgeneration region within the transfer nip, and there will be asignificant voltage drop across the adhesive in the transfer nip. Asknown in the art of electrostatics, the voltage drop across the highresistivity “insulating” adhesive will increase with increasingdielectric thickness D_(OA) of the overspill. This reduces the voltagedrop across the toner and hence reduces the transfer field in theoverspill region. With too high a dielectric thickness D_(OA) thetransfer field perturbation in the overspill region due to the highresistivity “insulating” overspill exceed the 10% level that istypically preferred for an imageable seam intermediate transfer belt.However, if the “nip charge relaxation time” of the overspill,T_(ρnipOA) is smaller than the characteristic effective “nip dwell time”T_(dnip) for the transfer system, the voltage drop across the overspillwill be small. Thus the resistivity condition defined by the overspillcondition where the “nip charge relaxation time” is smaller than theeffective transfer “nip dwell time”, T_(ρnipOA)<<T_(dnip), is mostpreferred in imageable seam intermediate transfer belt systems havingsignificant adhesive overspill. This is significant because adhesiveoverspill is beneficial in that it increases the seam strength.

Although semiconductive overcoats in the resistivity ranges discussedabove are useful and preferred for most imageable seam intermediatetransfer systems, imageable seam systems can also have relatively higherresistivity overcoats and seam adhesive materials than that discussedabove, with some constraints. In some intermediate transfer systems theuse of higher resistivity overcoats has some advantages. For example,relatively high resistivity materials having good toner releaseproperties and low cost are often more available than materials havingsome degree of electrical control. As another example, relatively highresistivity overcoats having high dielectric strength can substantiallyeliminate shorting issues, even when the intermediate belt substratelayer is relatively conducting. This is useful in systems that usesubstrate layer that has the proper resistivity at low applied fieldsbut has an undesirably low resistivity at high applied field conditions(say 500 to 1000 volts drop across the belt). A sufficiently highresistivity overcoat can reduce the shorting issues in the transfer nipby increasing the composite resistance in the transfer nip.

If a “cyclic charge relaxation time”, T_(ρcyC), of the overcoat is muchlarger than a characteristic “cyclic dwell time”, T_(dcy), for theintermediate transfer system, then the overcoat will begin to behavelike an “insulator” during the cycle dwell time. Then, charge will buildup on the “insulating” overcoat after each transfer zone. This chargebuildup can cause transfer problems in subsequent transfer zones if thevoltage drop across the overcoat is too high. Also, charge deposition onthe overcoat side after passing through transfer zones is generally dueto air breakdown in the transfer zones and can be somewhat non-uniform.This can cause further transfer problems with very high resistivityovercoats, especially if the voltage drop across the overcoat is large.However, it is known in the art of electrostatics that the voltage dropacross the overcoat is proportional to the dielectric thickness of theovercoat, D_(C). Therefore, a low dielectric thickness overcoat canreduce the transfer problems related to very high resistivity overcoat.Furthermore, the uniformity and magnitude of the charge on an overcoatcan be improved somewhat by using corona charge leveling devices knownin the art, such corotrons or scorotrons. Thus the combination of a“sufficiently small” coating dielectric thickness, typically D_(C)<25microns and more preferably less than around 10 microns, and the use ofcharge neutralizing devices can enable the use of relatively insulatingcoatings.

If more than one overcoating layer is applied to an imageable seamintermediate transfer belt, the properties of each layer needs to beconsidered. The sum of the contributions of the individual layers on theeffective dielectric thickness of the composite overcoat should meet thepreferred dielectric thickness levels. For example, ifT_(92 cyC)>>T_(dcy), applies for all of the layers, then all of thelayers behave “insulating” and the dielectric thickness values discussedabove apply to the “sum of the dielectric thickness” of each of theindividual layers. The sum of the individual dielectric thickness(thickness divided by dielectric constant) for the layers shouldtypically be less than around 25 microns and more preferably should beless than around 10 microns for a high resistivity dielectric thicknessovercoat in a multiple color intermediate transfer system. With multiplelayer overcoats, it is also possible that some of the layers have highenough resistivity to behave “insulating” while some of the layers mayhave a low enough resistivity that no significant voltage drop acrossthat layer's thickness occurs. If the condition T_(ρcyC)>>T_(dcy)applies for any of the layers, that layer behaves relatively insulatingduring the cyclic dwell time and that layer's dielectric thicknessshould be added to the total effective dielectric thickness. If thepreviously discussed condition T_(ρnipC)<<T_(dnip) applies for any otherlayer, that layer will have substantially no voltage drop across itafter the cyclic dwell time and that layer's dielectric thickness shouldbe taken as effectively zero for purposes of the previously discussedtransfer nip issues caused by high dielectric thickness. Conditionsbetween these extremes follow from these examples.

It is important to choose a seam adhesive that has electrical propertiesthat are in “good correspondence” to the electrical properties of thesubstrate layer. Good correspondence does not mean “the same” electricalproperties. Rather, good correspondence implies that the electricalproperties produce sufficiently low field perturbations around seam toallow toner to be transferred onto and off of the seam region withoutsignificant degradation of the transferred image. As discussedpreviously, typically this means that the transfer field in the seamregion should be within 20%, and more preferably it should be within10%, of the transfer field in regions away from the seam.

To understand good correspondence it is useful to use the previouslydescribed characteristics of “nip charge relaxation times” andcharacteristic “nip dwell times.” The desired resistivity relationshipsbetween the substrate and the adhesive depend on various systemparameters that are best determined from these characteristic times. Thenip charge relaxation time of the substrate far from the seam gap,T_(ρnipS), is of interest because this will influence the transferfields that are present “far” from the seam. Typically, “far” from theseam will usually mean distances from the seam along the belt surfacethat are much greater than the size of the seam region that hasperturbed electrical properties relative to the far region. For example,in a puzzle cut imageable seam if the adhesive in the seam kerf gap hasperturbed electrical properties relative to the substrate and thesurrounding substrate puzzle cut “petals” have the same electricalproperties as the substrate material far from the seam, “far” will meandistances much larger than the puzzle cut kerf gap. On the other hand,if the electrical properties of the surrounding substrate puzzle cutpetals or nearby seam regions are perturbed relative to the far region,“far” will mean distances much larger than the size of such perturbedregion. Such perturbations of the surrounding or nearby substrateregions of the seam can sometimes occur for example due to chemical,mechanical or other seam processing parameters such as local heatingthat might be used to achieve a good seam joining adhesion. At “far”distances from the perturbed electrical region of the seam, the transferfields perturbations due to the perturbed electrical properties of theseam region are generally small. The parameter T_(ρnipS) is thecharacteristic charge relaxation time it takes in the transfer nip forthe voltage across the substrate layer thickness to drop due toconduction of charge across the substrate thickness. The approximateexpressions for T_(ρnipS) are the same as the ones described during thediscussion of charge decay across the coating thickness. The substrateresistivity ρ_(s), dielectric thickness D_(s) and dielectric constantK_(s) are now substituted for the corresponding coating propertiespreviously discussed. Previous discussions of the influence of fielddependent resistivities also apply here for both the substrate and theadhesive materials.

The transfer of toner onto an imageable seam intermediate transfer beltis explained with the assistance of FIG. 8, which illustrates aquasi-electrostatic situation within a transfer nip 140. As shown, aphotoreceptor comprised of a ground conductor 280 and a photoconductivesurface 282 holds a toner layer comprised of toner particles 284.Separated from the toner layer by an air gap 286 is an imageable seamintermediate transfer belt 41 (reference FIG. 6) that rides on aconductive roll 288. The transfer fields in the seam region areinfluenced by the characteristic seam relaxation time T□gap. This is thecharacteristic time it takes for charge to flow across the adhesive 48in the seam gap 20. The description of the seam gap charge relaxationtime T□gap is somewhat more complex than for the substrate region farfrom the seam because the dimensions of the seam gap are typicallycomparable to the thickness of the substrate. Simple parallel plateapproximations can often be used for the approximate relaxation times ofthe intermediate materials layers far from the seam, but this simpleapproximation does not apply around the seam gap. The characteristic nipcharge relaxation across the seam gap is still proportional to theadhesive resistivity. However, the nip charge relaxation time for theadhesive in the small seam gap region is influenced somewhat by thesurrounding substrate properties and by the geometry of the seam. Itgenerally needs to be determined using numerical calculations ormeasurements.

If the substrate nip charge relaxation time far from the seam is muchsmaller than the nip dwell time, that is if T_(ρnipS)<<T_(dnip), therewill be substantially no voltage drop across the substrate 10 during thedwell time in the transfer nip in belt regions far from the seam(ΔV_(S)=0). This is due to conduction across the substrate during thenip dwell time. On the other hand, if the charge relaxation time for theadhesive in the seam gap region is much larger than the nip dwell time,that is if T_(ρgap)>>T_(dnip), then the adhesive 48 begins to behavelike an “insulator” during the transfer nip dwell time. Then, there canbe a significant voltage drop ΔV_(gap) across the adhesive in the seamgap during the dwell time. Thus the voltage drop across the intermediatetransfer belt will be somewhat higher in the seam region than in regionsaway from the seam region. Therefore, it follows that the transfer fieldwill be lower in the seam gap region than in the regions away from theseam. As explained later, whether or not the electrical properties arein “good correspondence” for this case can depend on factors such as thedielectric constant of the adhesive material, K_(A), the kerf gap width,and the overcoating thickness.

If the substrate nip charge relaxation time far from the seam is muchlarger than the nip dwell time, that is if T_(ρnipS)>>T_(dnip), therewill then be a voltage drop ΔV_(S) across the substrate during the dwelltime in the transfer nip in regions far from the seam. The voltage dropacross the substrate is proportional to the dielectric thickness D_(S)of the substrate. However, if the charge relaxation time for theadhesive 48 is much smaller than the nip dwell time, that is ifT_(ρgap)<<T_(dnip), then due to conduction there will be substantiallyno voltage drop across the adhesive during the dwell time (ΔV_(gap)=0).In this case, it follows that the transfer field will be somewhat higherin the seam gap region than in regions far from the nip. The adhesiveelectrical properties are thus typically not in “good correspondence”with the substrate electrical properties. Whether or not the electricalproperties are in “good correspondence” can depend on the dielectricconstant of the substrate material, the kerf gap, and the overcoatthickness.

If the substrate nip charge relaxation time far from the seam is muchsmaller than the nip dwell time, that is if T_(ρnipS)<<T_(dnip), therewill again be substantially no voltage drop across the substrate(ΔV_(S)=0) during the dwell time in belt regions far from the seam. Now,if the charge relaxation time for the adhesive in the seam gap region isalso much smaller than the nip dwell time, that is ifT_(ρgap)<<T_(dnip), then there will also be substantially no voltagedrop (ΔV_(gap)=0) across the adhesive during the dwell time in thetransfer nip. In this case, the voltage drop across the seam gap regionand the voltage drop across the regions of the substrate far from theseam are about the same (nearly zero) in the transfer nip. So, thetransfer fields E_(far) and E_(gap) in these two regions aresubstantially the same. In this case, the adhesive and substrateelectrical properties are within the preferred conditions of “goodcorrespondence.” Note that in this instance the electrical properties ofthe adhesive and the substrate can be very different and still be in themost favorable regime of “good correspondence.” Mainly to be in “goodcorrespondence” in the resistivities of the seam adhesive and thesubstrate can be significantly different as long as both are alwaysbelow a threshold level. Of course, as previously discussed, anintermediate transfer system can also have further constraints on thelower limit of the resistivity of the substrate and adhesive materials,due typically to “shorting” and lateral conduction problems. So, to bein “good correspondence” in systems subject to “shorting” and lateralconduction problems, the resistivities of the seam adhesive and thesubstrate should be below the values defined by the charge relaxationtimes, and they should also typically be above around the shorting andlateral conduction threshold values for the system.

To estimate the charge relaxation time for the seam gap region referagain to FIG. 8. The bottom of an intermediate transfer belt in the seamregion of a transfer nip is assumed to be suddenly switched from groundpotential to a fixed bias potential at time=0. The substrate andadhesive materials can then be treated as “leaky dielectrics” having aresistance and capacitance in parallel. This is a good approximation forthe electrical behavior of typical intermediate transfer materials intransfer nips. The voltage drop across the center of the seam can benumerically calculated as a function of time after the voltage isapplied to allow an estimate of the nip charge relaxation T_(ρgap). Forseam gaps large in comparison to the substrate thickness the chargerelaxation time for the adhesive can be approximated by the simpleparallel plate formula: T_(ρA)=K_(A)ρ_(A)ε₀[1+D_(A)/ΣD_(I)]. Indeed, thesimple parallel plate approximation can often be used even for smallgaps.

At any rate, the charge relaxation time T_(ρgap) can be estimatednumerically. As examples, an effective transfer nip width of 0.20 inchesand a belt speed of 10 in/sec yields a nip charge relaxation time ofT_(dnip)=0.020 seconds. Then, adhesive resistivities of around ≦2×10¹⁰ohm-cm will achieve the condition T_(ρgap)<<T_(dip). Another example, ifthe belt speed is decreased to 2.0 in/sec the dwell time is T_(dnip)=0.100 seconds. The condition T_(ρgap)<<T_(dnip) would then occur atadhesive resistivities of around ≦1×10¹¹ ohm-cm. For many systems, thecondition T_(ρgap)<<T_(dnip) will typically occur for adhesiveresistivities near or below the around the 10¹⁰ ohm-cm resistivityrange. However, this should be estimated for each specific system. Thusthis “good correspondence” condition is mainly a condition of arelatively semiconductive substrate with a relatively semiconductiveadhesive.

As discussed, the conditions T_(ρnipS)<<T_(dnip) and T_(ρgap)<<T_(dnip)are a preferred regime for good correspondence where the transfer fieldsare substantially the same in the seam and in regions far from the seam.However, in order for the electrical properties of the substrate andadhesive to be in good correspondence under all situations theseconditions need to occur over the full range of variability of thesubstrate and adhesive electrical properties. For example, theconditions need to apply despite changes in the environment,manufacturing tolerance, and material aging conditions that may occur inthe intermediate transfer system. Fortunately, the conditionsT_(ρnipS)<<T_(dnip) and T_(ρgap)<<T_(dnip) for good correspondence canallow significant tolerance of an imageable seam intermediate transferbelt despite differences in the electrical properties of the twomaterials. For example, in an imageable seam intermediate system where“shorting” issues require ≧10⁷ ohm-cm for the intermediate transfer beltmaterials, the substrate and adhesive resistivities can be substantiallyanywhere within the tolerance range of 10⁷ to 10¹⁰ ohm-cm. To avoidlateral conduction issues, the lateral resistivity should typically beabove 10⁸ ohms/square, preferably above 10¹⁰ ohms/square. In summary,the “good correspondence” imageable seam substrate and adhesiveelectrical property conditions defined by T_(ρnipS)<<T_(dnip) andT_(ρgap)<<T_(dnip) are most favorable due to high tolerance fordifferences in the substrate and adhesive resistivity.

In general, the substrate resistivity condition defined byT_(ρnipS)<<T_(dnip) is a most favorable one for imageable seamintermediate transfer belts. This substrate condition can even allowwider tolerance to the adhesive resistivity if the dielectric constantof the adhesive material is above a critical value. For example, thissubstrate resistivity condition can allow the adhesive material to besubstantially “insulating” during the dwell time of the transfer nipwhile still achieving the desired “good correspondence” condition. Tounderstand this, note that a relatively insulating adhesive causes somevoltage drop across the adhesive, but the preferred substrate conditionhas substantially no voltage drop during the transfer nip dwell time.This is a fundamental cause of the perturbation of the transfer field inthe seam region. However, as is well known in the art of electrostatics,the voltage drop across the “insulating” adhesive in the seam gap alsodecreases with increasing adhesive dielectric constant. Therefore, itfollows that if the dielectric constant of the adhesive is sufficientlylarge, the resulting voltage drop across the adhesive in the gap can bemade sufficiently small to achieve the desired less than 10% fieldperturbation in spite of the high adhesive resistivity. For example,consider a relatively insulating adhesive (10¹² ohm-cm;T_(ρgap)>>T_(dnip)). If a substrate defined by the conditionT_(ρnipS)<<T_(dnip) is used, then if the insulating adhesives has adielectric constant K_(A)>12 the desired <10% field perturbation isachieved when the kerf is around 25 micons. It follows from priordiscussions that with lower kerf than 25 microns the desired <10% fieldperturbation can be achieved using somewhat lower K_(A) than 12.Further, for systems can tolerate field perturbations somewhat higherthan 10%, good correspondence can be obtained with lower K_(A). Still,imageable seam intermediate belt systems wishing to operate under theconditions of a relatively insulating adhesive discussed above willtypically prefer the seam region to have a K_(A) greater than about 5.

Another constraint on the upper limit of the adhesive resistivity iscyclic charge buildup. Cyclic charge buildup occurs if the adhesiveresistivity ρ_(A) is so high that it interferes with subsequenttransfers. To prevent this the adhesive cyclic charge relaxation timeshould be less than the cyclic dwell time between transfers(T_(ρcyA)<<T_(dcy)). However, this still adds significant extratolerance for the seam adhesive resistivity. For example, fromextensions of previous estimates the desired adhesive resistivity for animageable seam should typically be below around 10¹³ ohm-cm for mostsystem conditions and should preferably be below around 10¹² ohm-cm forhigh process speed systems having small distances between imagingstations.

In summary, the “good correspondence” condition to achieve typicallyacceptably low field perturbations for the semiconductive imageable seamsubstrate defined by T_(ρnipS)<<T_(dnip) can allow wide tolerance foradhesive resistivity if the adhesive resistivity is sufficiently low(T_(ρgap)<<T_(dnip)) and even wider tolerance for adhesive resistivity(up to an seam gap T_(ρcy)<<T_(dcy)) if the adhesive dielectric constantis moderately high, typically K_(a)>5.

A similar dielectric constant effect can occur for the unfavorableelectrical property correspondence: T_(ρnipS)>>T_(dnip) andT_(ρgap)<<T_(dnip). Here the substrate resistivity is high enough for itto be substantially an “insulator” during the transfer dwell time butthe adhesive has a low enough resistivity so that there is substantiallyno voltage drop across the seam gap. Similar to the above discussion,the voltage drop across the substrate will get smaller as the substratedielectric constant gets larger. Estimates of the field perturbation forthis case as a function of the substrate dielectric constant K_(S)suggest that, in order to achieve the desired <10% field perturbation,very high Ks is desired. The desired K_(s) for low field perturbation,and hence for acceptably good electrical property correspondence, cantypically be greater than around 25 under some extreme conditions of avery thin overcoating layer such as a 5 micron thick layer, and with acondition of very large mismatch of the substrate and seamresistivities. The desired K_(s) to achieve good electrical propertycorrespondence for this case decreases with for example increasingovercoating thickness, but the desired K_(s) is typically greater thanaround 5 for most systems.

Another “high resistivity” substrate case is the condition where:T_(ρgap)>>T_(dnip) and T_(ρnipS)>>T_(dnip). Under this condition thecharge relaxation times for the substrate and adhesive are both muchgreater than the nip dwell time over the full range of materialsvariability. However, this is not a sufficient condition for insuringgood correspondence. In this case the substrate and the adhesive actsubstantially like “insulators” during the dwell time of the transfernips. When materials act like insulators during the transfer nip dwelltime the voltage drop across the belt is proportional to the dielectricthickness of the belt materials. Due to this, good correspondenceincludes the constraint that the dielectric constants of the adhesiveK_(a) and the substrate K_(s) are similar, typically within about 30%,and most preferably the dielectric constants are substantially the same.Also, even further constraints are needed for good correspondence. Inparticular, the resistivity of both the substrate and adhesive need tobe chosen so as to avoid different amounts of cyclic charge buildup onthe substrate and the adhesive between transfer stations. Otherwise, thedifferent cyclic charge buildup in the seam region compared to regionsaway from the seam can cause field perturbations for subsequent tonertransfers. There are two basic ways of addressing this problem.

The preferred way is for the substrate and adhesive to both havesufficiently low resistivity that discharge occurs between transferstations. From analogy with previous discussions, the condition desiredis T_(ρcy)<<T_(dcy) for both the substrate and the adhesive, where thecyclic charge relaxation time for both the substrate and the adhesive ismuch smaller than the cyclic dwell time between subsequent transferstations. An alternative condition is where both the substrate and theadhesive resistivities are high enough so that the same cyclic chargebuildup will occur on both the substrate and seam adhesive. While cycliccharge buildup is generally not desired, it can be acceptable withproper constraints. Having similar cyclic charge buildup betweentransfer stations on both the substrate and the adhesive will at leastprevent field perturbations in subsequent transfer stations. Fromanalogy to previous discussions, a necessary condition for similarcyclic charge buildup is T_(ρcy)>>T_(dcy) for both the substrate and theadhesive. Also, the substrate and adhesive dielectric constants shouldbe similar, and high dielectric constant substrate and adhesive areusually needed to avoid transfer problems associated with highresistivity, high dielectric thickness intermediate materials.

It can be inferred from all of the above discussions that highresistivity substrate materials (T_(ρnipS)>>T_(dnip)) can allowimageable seam conditions. However, for the reasons discussed,substrates having electrical properties in the range T_(ρnipS)<<T_(dnip)are most preferred for imageable seam intermediate transfer beltsystems.

Other conditions for the intermediate substrate electrical propertiescan make it more difficult to achieve desired “good correspondence”between the substrate and adhesive electrical properties for producingthe desired low field perturbations with an imageable seam. For example,a difficult substrate condition for an imageable seam can occur when theresistivity of the substrate varies between conditions where thesubstrate charge relaxation time is sometimes shorter than and sometimeslonger than the characteristic dwell times. Consider a case where thesubstrate resistivity under one set of extreme conditions may be lowenough to have T_(ρnipS)<<T_(dnip) so that there is substantially novoltage drop across the substrate in the transfer nip dwell time forthat extreme condition. Such an extreme condition might occur, forexample, with substrates at the low resistivity end of the manufacturingtolerance and when the RH is high. If the substrate resistivity at anopposite set of extreme conditions is high enough so that the conditionT_(ρnipS)>>T_(dnip) occurs, there will be a voltage drop across thesubstrate at this other extreme condition.

Ideally, the nominal adhesive electrical properties are relatively closeto the substrate electrical properties within manufacturing tolerancesand have similar response to environment, aging, and applied fieldfactors. Otherwise, the adhesive and substrate materials can easily moveaway from the desired “good correspondence” conditions. One way ofincreasing the tolerance of an imageable seam intermediate belt systemto differences in the electrical properties of the substrate andadhesive is to utilize a “sufficiently thick” overcoat. The use of asufficiently thick overcoat can allow some of the less favorableconditions discussed above, such as the condition T_(ρnipS)>>T_(dnip)for the substrate while the adhesive material is at the conditionT_(ρnipA)<<T_(dnip).

Overcoats can significantly reduce the perturbations of the transferfields caused by poor matching of the electrical properties of the seamgap adhesive compared to the electrical properties of the substrate. Itis the fields in the toner layer that drive toner transfer. An advantageof an overcoat is that it moves the seam gap further away from the tonerlayer. It is well known in the art of electrostatics that the effect onelectrostatic fields of a local perturbing factor typically reduces withdistance away from the perturbing factor. So, moving the fieldperturbing seam gap further away from the toner layer can greatly reducethe perturbations in the transfer field acting on the toner that wouldotherwise occur if, for example, the seam adhesive electrical propertiesare too highly mismatched compared to the electrical properties of thesubstrate. Generally the good effect of the overcoat on minimizingtransfer field perturbations will increase with increasing overcoatingthickness. So, a sufficiently thick overcoat can enable imageable seamsystems that may wish to use highly mismatched seam adhesive andsubstrate electrical properties. Smaller kerf gap can also be anadvantage over large kerf gaps in that the perturbing effect of the seamgap will also generally decrease quicker with distance away from the gapwith smaller kerf gaps compared to larger gaps.

To estimate the desired overcoating properties for allowing highlymismatched adhesive and substrate electrical properties, the effects ofthe properties of the overcoating on the transfer fields needs to beestimated. The effect of the overcoating can be estimated using thequasi-static electrostatic numerical simulations similar to thatdiscussed previously for estimating nip charge relaxation times. Forexample, assume a substrate resistivity of 10⁸ ohm-cm and a niprelaxation time T_(ρsub) of around 7×10⁻⁵ sec. With an adhesiveresistivity of 10¹² ohm-cm the adhesive gap nip relaxation time T_(ρgap)can be estimated to be around 0.7 seconds. With a nip dwell time of 0.01seconds, the adhesive electrical properties are highly mismatched to thesubstrate electrical properties: T_(ρnipS)<T_(dnip); T_(ρgap)>>T_(dnip).In this case the substrate can be considered to be “sufficientlyconducting” during the nip dwell time that the voltage drop across thesubstrate thickness is negligible during the nip dwell time. On theother hand, the adhesive layer in the seam gap acts relatively“insulating” during the nip dwell time so there is some voltage dropacross the seam adhesive thickness during the nip dwell time. Therefore,the transfer fields in a small air gap are perturbed by the mismatchedelectrical properties with the transfer fields in the seam gap regionbeing smaller than the fields far from the seam. The air gap transferfields can be estimated from numerical electrostatic analysis. Ofinterest is the field perturbation percentage P:P=100[abs(E_(far)−E_(gap))]/E_(far). The parameter P is the seam fieldperturbation, that is the absolute value of the percentage difference ofthe transfer field in regions far from the seam gap compared to thetransfer field in the center of the seam gap. As discussed, for animageable seam intermediate transfer belt P should typically be lessthan 20% for most systems and preferably P should be less than 10% forsome systems.

As the thickness of the overcoat, d_(c) increases, the fieldperturbations decrease. Assuming a 25 micron wide kerf acceptable fieldperturbations usually can be achieved with overcoatings about 12 micronsthick. Generally, smaller kerfs can allow thinner overcoatings such as 5microns. Typically, acceptably low field perturbations will occur whenthe coating thickness is comparable to or thicker than the seam kerfgap. The beneficial effect on the transfer field perturbations occursover a relatively wide range of overcoating resistivity. Generally lowerresistivity overcoats result in lower field perturbations. In general,an overcoating should have a low enough resistivity to avoid cycliccharge issues without additional cyclic neutralizing devices. That is,the overcoating should have the condition T_(ρcyC)<<T_(dcy). Thisovercoating resistivity also enables a relatively wide mismatch in theadhesive and substrate electrical properties.

An imageable seam intermediate transfer belt that uses a substratematerial having electrical properties within the preferredsemiconductive condition (T_(ρnipS)<<T_(dnip)) can tolerate bothrelatively insulating and sufficiently conducting adhesive materialswith the addition of a “sufficiently thick” overcoating as definedabove. That is, the overcoat allows the seam gap adhesive to behaverelatively “insulating” in the transfer nip (T_(ρnipA)>>T_(dnip))without causing unacceptable transfer field perturbation. Again, asemiconductive substrate defined by the condition (T_(ρnipS)<<T_(dnip))is most preferred for imageable seam intermediate transfer belts.

One further comment should be made regarding insulating adhesives. Ifthe charge relaxation time for the adhesive is longer than the dwelltime between transfer stations, then charge can accumulate on thebackside of the adhesive in the seam gap. If allowed to accumulate, theadhesive charge can interfere with subsequent transfers. Therefore, ifthe seam adhesive resistivity is such that T_(ρcy)>>T_(dcy), a chargeneutralizing approach for the seam adhesive will be needed for the backof the belt. This can be done using simple contact static eliminationdevices, such as by using grounded contact brushes. Even more preferred,the adhesive resistivity is ideally kept sufficiently low such thatT_(ρcy)<<T_(dcy), and then such discharging devices are not needed. Fromprevious estimates, the desired adhesive resistivity is typically lessthan 10¹³ ohm-cm for most systems and preferably it is less than 10¹²ohm-cm for many systems.

Another mismatch condition is a relatively insulating substrate and arelatively conducting adhesive, for example, a substrate having a highresistivity such that the nip relaxation time T_(ρnipS) is around 0.7seconds, and an adhesive having a low resistivity such that the adhesivegap nip relaxation time T_(ρgap) is around 7×10⁻⁵ seconds. Assuming anip dwell time of 0.01 seconds, this represents a highly mismatchedsubstrate and adhesive electrical property condition:T_(ρnipS)>>T_(dnip); T_(ρgap)<<T_(dnip). The substrate now behavessubstantially like an “insulator” during the dwell time of the transfernip so that there is a voltage drop across the substrate during thetransfer nip dwell time. However, the adhesive in the seam gap now actslike a “conductor” during the transfer nip dwell time, in that there isessentially no voltage drop across the seam gap adhesive layer duringthe transfer nip dwell time. In this case, the transfer fields arelarger in the seam gap region compared to regions far from the seam. Amuch higher coating thickness is typically needed to “hide” the effectsof the highly mismatched adhesive and substrate electrical propertiesthan the case where the substrate is relatively conducting and theadhesive is relatively insulating. Mainly, this is because conductionthrough the relatively conductive adhesive drives the top of theadhesive layer to the applied potential, and this moves the source ofthe field perturbation closer to the toner layer.

At any rate, when the substrate resistivity is relatively high(T_(ρnipS)>>T_(dnip)) and the adhesive resistivity is relatively low(T_(ρgap)<<T_(dnip)), a coating thickness above about 150 micorns can beneeded to achieve the preferred field perturbation of less than 10% foran imageable seam when the seam kerf gap is around 25 microns and theovercoating has a resistivity near 10¹² ohm-cm. Again, factors such as alower resistivity overcoating or lower kerf gap size can reduce therequired overcoating thickness. However, typically the minimumovercoating thickness desired for reducing field perturbations in thiscase is typically larger than the size of the kerf and it is preferablyat least three times as the kerf size.

The volume resistivity of a coating can in general have differentresistivity in the lateral direction and in the thickness direction ofthe coating. Independent of the volume resistivity in the thicknessdirection of the overcoating, a sufficiently low lateral resistivity foran overcoating can help to reduce the field perturbations that wouldotherwise be caused by mismatch of the electrical properties of the seamregion and the far regions of the imageable seam intermediate belt. Thisis because lateral conduction in the seam region will tend to smooth outany tendency for voltage drops along the belt surface at the tonertransfer interface with the imageable seam intermediate belt. Asufficiently low lateral resistivity overcoating can also be beneficialfor reducing the tendency for toner disturbances that can occur whensubstrate materials having very high lateral resistivity are used withimageable seam intermediate transfer systems. For example, non uniformcharge patterns can form on an intermediate transfer belt due to nonuniform charge exchange between the near the transfer nips, and this canlead to a redistribution of the transferred toner in patterns that aretypically referred to as “toner disturbance” defects. When the substratelateral resistivity is somewhat below around 10¹² ohms/square, thesenon-uniform charge patterns can be dissipated via lateral conductionbetween subsequent transfer stations and this can reduce the tonerdisturbance problems. Even if the substrate lateral resistivity issomewhat above around 10¹² ohms/square, in many systems tonerdisturbance problems can be reduced with imageable seamed intermediatetransfer belts if the overcoating used has a lateral resistivity belowaround 10¹² ohms/square. The desired condition depends on details of thetransfer system geometry and process speed conditions. Preferably forsome systems the overcoating should be near or below around 10¹¹ohms/square for this, and more preferably it should be near or belowaround 10¹⁰ ohms/square when high process speed conditions are present.These same ranges of low overcoating lateral resistivity conditions arealso desirable for reducing field perturbations caused by largeelectrical property mismatch conditions between the seam and far regionsof the intermediate transfer belt via lateral conduction along theovercoating in the seam region.

If the lateral resistivity of the composite overcoated intermediate beltis below a threshold condition, significant charge conduction can occurlaterally along the belt during the dwell time that a section of theintermediate belt takes to travel through the pre and post transfer nipregions of the transfer zone. The composite lateral resistivity can betaken to mean the lateral resistivity determined by treating themultiple layer belt as an equivalent composite single layer. Thethreshold lateral resistivity condition for the composite belt increaseswith factors such as with increasing process speed and with increasingdistance between the transfer field generating device and the start ofair gaps between the belt and the toner layers in the pre and post nipregions of the transfer system. At typical transfer geometry and processspeeds significant lateral conduction effects can occur if the compositebelt lateral resistivity is below about 10¹⁰ ohms/square. In the lowlateral resistivity condition the electrostatic fields in the pre andpost nip regions of the transfer zones can be affected by the lateralresistivity, and this can in turn cause a dependence of transfer on thelateral resistivity. Therefore, with an imageable seamed intermediatetransfer belt, if the lateral conductivity of the composite belt in farregions away from the seam region of the belt is below around 10¹⁰ohms/square, the electrical properties of the composite overcoatedintermediate belt should be chosen to have substantially the samelateral resistivity in the seam region as in the far regions away fromthe seam. Some small difference can typically be allowed depending onfactors such as the toner adhesion characteristics and on the acceptableamount of field perturbation that can be tolerated by the system beforedeclaring an unacceptable level of difference between the transferredimage in the seam and far regions. However, typically the lateralresistivity of the composite imageable seamed intermediate belt in theseamed region should be within about a factor of four of the lateralresistivity of the composite imageable seamed intermediate belt in farregions beyond the seam region when the lateral resistivity of thecomposite belt is below about 10¹⁰ ohms/square in the far regions.

From all of the above discussions it follows that sufficiently thickovercoats having optimized resistivity enables a very wide mismatch inthe electrical properties of the substrate and the adhesive. However, ifa very thick overcoating is used the overcoating resistivity should becontrolled to reduce transfer problems. Typically, if the overcoatingdielectric thickness is above around 25 microns, the overcoatingresistivity should preferably be low enough so that the nip chargerelaxation time will be lower than the transfer nip dwell time to avoidhigh voltage drop across the overcoating during the transfer nip dwelltime. It can be inferred from past discussions that the preferredovercoating resistivity will typically be below around 10¹⁰ ohm-cm forsuch high overcoating thickness cases.

In addition, the intermediate transfer member 41 should have sufficientlateral stiffness to avoid registration issues due to elastic stretch.Stiffness is the sum of the products of Young's modulus times the layerthickness for all of the layers of the intermediate transfer member. Thepreferred range for the stiffness depends on various systems parameters.The required value of the stiffness increases with increasing amount offrictional drag at and/or between the toner image producing stations122, 124, 126, and 12828. The preferred stiffness also increases withincreasing length of the intermediate transfer member 41 between tonerimage producing stations, and with increasing color registrationrequirements. The stiffness is preferably >800 PSI-inches and morepreferably >2000 PSI-inches. Finally, it should be noted that inpractice an imageable seam intermediate transfer belt undergoessignificant mechanical stresses. Therefore, the seam should have a seamstrength of 15 pounds per linear inch or greater.

While this invention has been described in conjunction with a specificembodiment thereof, it is evident that many alternatives, modifications,and variations will be apparent to those skilled in the art. Forexample, while the foregoing describes a 2-belt dry powder transfusesystem, other systems, including single belt, multi-pass intermediatebelt systems, tandem intermediate belt systems, transfuse systems andliquid ink development systems, may benefit from the principles of thepresent invention. Furthermore, the forgoing generally describes the useof overcoated imageable seam belts. However, non-overcoated systems canalso be used and are preferable in some applications. Furthermore, theforegoing has included numerous architectural features that are optionalin specific applications. For example, pre-transfer treatment ofphotoconductors, corona toner charge treatment of the imageable seamintermediate belt prior to transfer, and the use of transfer assistblades while desirable in some applications, may not be desirable inother applications. Accordingly, it is intended to embrace all suchalternatives, modifications and variations that fall within the spiritand broad scope of the appended claims.

We claim:
 1. A marking machine, comprising: a moving photoreceptor belt;a charging station for charging said photoreceptor belt; an imagingstation for exposing said charged photoreceptor belt so as to produce alatent image; a developer for depositing toner on said latent image; atransfer station for transferring said deposited toner onto a substrate,said transfer station including a seamed intermediate transfer belt thatreceives toner from said imaging station; a fuser having a fusing memberfor receiving toner from said intermediate transfer belt and for fusingsaid transferred toner to said substrate; and a cleaning station forcleaning said photoreceptor; wherein said seamed intermediate transferbelt comprises a seamed substrate formed by joining ends of a belt at aseam, wherein said seamed substrate has a seam region around said seamand a far region away from said seam; and wherein said seam region hasgood electrical property correspondence with said far region, andwherein said seam and far regions have lateral resistivity greater than10⁸ ohms/square.
 2. A marking machine according to claim 1, wherein saidfar region has a bulk resistivity between 10⁷ and 10¹⁰ ohm-cm and saidseam region has a bulk resistivity between 10⁷ and 10¹³ ohm-cm.
 3. Amarking machine according to claim 1, wherein said far region has a bulkresistivity between 10¹⁰ and 10¹³ ohm-cm and the bulk resistivity ofsaid seam region is between 5×10⁹ and 10¹³ ohm-cm.
 4. A marking machineaccording to claim 1, wherein said far region and seam region have abulk resistivity greater than 10¹³ ohm-cm, the dielectric thickness ofthe said far region is no greater than 25 microns, and the dielectricthickness of said far region and seam region are within 20% of eachother.
 5. A marking machine according to claim 1, wherein said farregion has a bulk resistivity between 10¹¹ and 10¹³ ohm-cm and the bulkresistivity of said seam region is between 5×10¹⁰ and 10¹³ ohm-cm.
 6. Amarking machine according to claim 1, wherein said seamed intermediatetransfer belt includes an overcoat having a toner bearing surface.
 7. Amarking machine according to claim 6, wherein said overcoat has a bulkresistivity less than 10¹³ ohm-cm.
 8. A marking machine according toclaim 6, wherein said overcoat has a lateral resistivity less than 10¹²ohms/square.
 9. A marking machine according to claim 6, wherein saidovercoat has a bulk resistivity less than 10¹³ ohm-cm.
 10. A markingmachine according to claim 6, wherein said overcoat has a bulkresistivity between 10⁸ and 10¹² ohm-cm.
 11. A marking machine accordingto claim 6, wherein said overcoat has a dielectric thickness greaterthan 25 microns and a bulk resistivity of said overcoating is notgreater than 10¹⁰ ohm-cm.
 12. A marking machine according to claim 6,wherein said substrate has a lateral resistivity above 10¹² ohms/squareand wherein said overcoat has a lateral resistivity less than 10¹¹ohms/square.
 13. A marking machine according to claim 6, wherein saidovercoat has a thickness greater than 5 microns.
 14. A marking machineaccording to claim 1, wherein said far region has a lateral resistivityless than 10¹⁰ ohms/square and said seam region has a lateralresistivity that is within a factor of four of the lateral resistivityof said far region.
 15. A marking machine according to claim 1, whereinsaid seam region has a bulk resistivity less than 10¹³ ohm-cm, whereinsaid far region has a bulk resistivity less than said bulk resisitivityof said seam region, and wherein said seam region has a dielectricconstant K greater than
 5. 16. A marking machine according to claim 1,wherein said seam region has a bulk resistivity less a bulk resistivityof said far region, and wherein said far region has a dielectricconstant K greater than
 5. 17. A marking machine according to claim 1,wherein said far region has a bulk resistivity less than 10¹³ ohm-cm.18. A marking machine according to claim 1, wherein said far region andsaid seam region both have a bulk resistivity less than 10¹³ ohm-cm. 19.A marking machine according to claim 1, wherein said seam region has abulk resistivity and a lateral resistivity within a factor of 5 of abulk resistivity and lateral resistivity of said far region.
 20. Amarking machine, comprising: a moving photoreceptor belt; a chargingstation for charging said photoreceptor belt; an imaging station forexposing said charged photoreceptor belt so as to produce a latentimage; a developer for depositing toner on said latent image; a transferstation for transferring said deposited toner onto a substrate, saidtransfer station including a seamed intermediate transfer belt thatreceives toner from said imaging station; a fuser having a fusing memberfor receiving toner from said intermediate transfer belt and for fusingsaid transferred toner to said substrate; and a cleaning station forcleaning said photoreceptor; wherein said seamed intermediate transferbelt includes: a seamed substrate formed by joining ends of a belthaving a top-side surface and a back-side surface along a kerf to form aseam, wherein said seamed substrate has a seam region around said kerfand a far region away from said kerf; an overcoat on said top-sidesurface, said overcoat having a toner-bearing surface; and wherein saidseam region has good electrical property correspondence with said farregion, and wherein said seam and far regions have lateral resistivitygreater than 10⁸ ohms/square.
 21. A marking machine according to claim20, wherein said overcoat has a thickness greater then twice the widthof said kerf.
 22. A marking machine according to claim 20, wherein abulk resistivity of said seam region is less then 10¹⁰ and bulkresistivity of said far region is greater than 10¹⁰, then said overcoathas a thickness greater then three times the width of said seam region.23. A marking machine according to claim 20, further including anadhesive within said kerf.
 24. A marking machine according to claim 23,wherein said adhesive extends over a surface of said substrate with aheight of less than 5 microns.
 25. A marking machine according to claim20, wherein said imageable seamed intermediate transfer belt has shortwavelength topological disturbances no greater than 10 microns.
 26. Amarking machine according to claim 20, wherein said imageable seamedintermediate transfer belt has long wavelength topological disturbancesno greater than 25 microns.
 27. A marking machine according to claim 20,wherein said seam has a mechanical seam strength greater than 15 poundsper linear inch.
 28. A marking machine according to claim 20, furtherincluding a kerf fill material having a resistivity less then 10¹⁰ andthat extends over said back-side surface, wherein said kerf fillmaterial has short wavelength disturbances less than 10 microns and longwavelength disturbances less then 25 micons.
 29. A marking machineaccording to claim 20, further including a kerf fill material thatextends over said top-side surface, wherein said kerf fill material hasshort wavelength disturbances less than 10 microns, and long wavelengthdisturbances less then 25 microns, and wherein said overcoating has athickness greater than said kerf fill material extension.
 30. A markingmachine according to claim 20, wherein said seam is formed byinterlocking elements.